The present invention relates to an inverter device which controls an electric motor by a sensorless vector system where any magnetic pole position sensor is not used.
When a brushless motor (an electric motor) is operated by sensorless vector control, heretofore, a voltage command value, an angular frequency and a phase have heretofore been calculated from a phase current flowing through an inverter main circuit, and a small and inexpensive shunt resistor has been used as a device for detecting this phase current. There are two types of a system using this shunt resistor: in one of them (a 1-shunt system), one shunt resistor is used as shown in FIG. 6 (see, e.g., Patent Document 1); and in the other system (a 2-shunt system), two shunt resistors are used to detect two-phase currents as shown in FIG. 1.
(1-Shunt System)
FIG. 6 shows a circuit constitution diagram of an inverter device 100 of the former 1-shunt system. Reference numeral 3 is an inverter main circuit of a three-phase pulse width modulation (PWM) system where a voltage supplied from a direct-current power source section 4 is converted into arbitrary variable voltages, e.g., three-phase pseudo alternating-current voltages having a variable frequency, output, and supplied to an electric motor (e.g., a synchronous motor) 6. That is, the inverter main circuit 3 comprises a switching element 7u of an upper arm for a U-phase, a switching element 8u of a lower arm for the U-phase, a switching element 7v of an upper arm for a V-phase, a switching element 8v of a lower arm for the V-phase, a switching element 7w of an upper arm for a W-phase and a switching element 8w of a lower arm for the W-phase, and a diode which allows the return of a current flowing through a winding wire of the electric motor 6 is connected in anti-parallel with each of the switching elements 7u, 8u, 7v, 8v, 7w and 8w. 
It is to be noted that in the switching element, an insulated gate bipolar transistor (IGBT) is used (this is similarly used hereinafter).
The switching elements 7u, 8u, 7v, 8v, 7w and 8w turn on when a pulse signal input into a base has an ‘H’ level, and turn off when the pulse signal has an ‘L’ level. Moreover, a shunt resistor 101 is connected to a direct-current bus, and the shunt resistor 101 has a constitution through which a direct-current bus current Idc (a shunt current) flows.
A controller 102 distributes the direct-current bus current Idc detected by the shunt resistor 101 to the respective phases based on pulse signals U, Ubar, V, Vbar, W and Wbar output by itself, to estimate three-phase currents flowing through the electric motor 6, i.e., a U-phase current Iu, a V-phase current Iv and a W-phase current Iw.
FIG. 7 shows the on/off state of each switching element of FIG. 6 and the direct-current bus current Idc (the shunt current) in one cycle (one carrier frequency) of a carrier wave used for the three-phase PWM system of the inverter device 100. For example, the direct-current bus current Idc is detected in the periods of circled 2 and 3 of FIG. 7.
In the period of circled 3, the switching element 7u of the upper arm for the U-phase turns on, the switching element 7v of the upper arm for the V-phase turns on and the switching element 8w of the lower arm for the W-phase turns on, whereby it is estimated that the W-phase current Iw (with a minus sign) is the direct-current bus current Idc detected in the period of circled 3.
In the period of circled 2, the switching element 7u of the upper arm for the U-phase turns on, the switching element 8v of the lower arm for the V-phase turns on, and the switching element 8w of the lower arm for the W-phase turns on, whereby it is estimated that the U-phase current Iu (with a minus sign) is the direct-current bus current Idc detected in the period of circled 2.
Moreover, the sum of the U-phase current Iu, the V-phase current Iv and the W-phase current Iw is zero, and accordingly, the U-phase current Iu is also estimated.
The controller 102 calculates the voltage command value, an estimated angular frequency value and the phase based on the phase of a rotating coordinate system and an angular frequency command value ω (a velocity command) by use of the estimated three-phase currents Iu, Iv and Iw (e.g., processing described in Patent Document 3), converts, into three-phase voltage command values, the voltage command value of the rotating coordinate system among these values, and further subjects the values to pulse width modulation, to output the pulse signals U, Ubar, V, Vbar, W and Wbar which control the switching elements 7u, 8u, 7v, 8v, 7w and 8w, respectively.
(2-Shunt System)
FIG. 1 shows a circuit constitution diagram of an inverter device 200 of the latter 2-shunt system. In the same manner as in the above system, reference numeral 3 is an inverter main circuit of a three-phase pulse width modulation (PWM) system where a voltage supplied from a direct-current power source section 4 is converted into arbitrary variable voltages, e.g., three-phase pseudo alternating-current voltages having a variable frequency, output, and supplied to an electric motor (e.g., a synchronous motor) 6. That is, the inverter main circuit 3 comprises a switching element 7u of an upper arm for a U-phase, a switching element 8u of a lower arm for the U-phase, a switching element 7v of an upper arm for a V-phase, a switching element 8v of a lower arm for the V-phase, a switching element 7w of an upper arm for a W-phase and a switching element 8w of a lower arm for the W-phase, and a diode which allows the return of a current flowing through a winding wire of the electric motor 6 is similarly connected in anti-parallel with each of the switching elements 7u, 8u, 7v, 8v, 7w and 8w. 
The switching elements 7u, 8u, 7v, 8v, 7w and 8w similarly turn on when a pulse signal input into a base has an ‘H’ level, and turn off when the pulse signal has an ‘L’ level. Moreover, in this case, shunt resistors 11 and 12 are connected to the lower arm for the U-phase and the lower arm for the V-phase, the shunt resistor 11 has a constitution through which a U-phase current Iu flows, and the shunt resistor 12 has a constitution through which a V-phase current Iv flows.
A controller 201 detects the U-phase current Iu from the shunt resistor 11, and detects the V-phase current Iv from the shunt resistor 12. Moreover, as described above, the sum of the U-phase current Iu, the V-phase current Iv and a W-phase current Iw is zero, and accordingly, the W-phase current Iw is estimated.
The controller 201 calculates the voltage command value, an estimated angular frequency value and the phase based on the phase of a rotating coordinate system and an angular frequency command value ω (a velocity command) by use of the detected and estimated three-phase currents Iu, Iv and Iw (the processing described in Patent Document 3), converts, into three-phase voltage command values, the voltage command value of the rotating coordinate system among these values, and further subjects the values to pulse width modulation, to output pulse signals U, Ubar, V, Vbar, W and Wbar which control the switching elements 7u, 8u, 7v, 8v, 7w and 8w, respectively.
Patent Document 1: Japanese Patent Application Laid-Open No. 2007-312511
Patent Document 2: Japanese Patent No. 3674578
Patent Document 3: Japanese Patent Application Laid-Open No. 2000-262088